[5.9] Parse init accessors

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Cherry-pick of #1731

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This is the SwiftSyntax-side change to swiftlang/swift#66283. Add parsing for init accessors:

struct Wrapper<T> {
  var wrappedValue: T
}

struct S {
  private var _value: Wrapper<Int>
  var value: Int {
    init(newValue) initializes(_value) {
      self._value = Wrapper(wrappedValue: newValue)
    }

    get { _value.wrappedValue }
    set { _value.wrappedValue = newValue }
  }
}

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@ahoppen Thank you for the help with cherry-pick!

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@swift-ci please test